Skizzik:

This is the main thing in my mod. He is based on the wither, but he has more heads and more power. He is 2 times stronger. He has more attacks and drops 10k experience on death.
The Skizzik has 500 hearts (1000 health points). He also has 5 stages and every stage he is more powerful and harder to beat.

Skizzik's Attacks:
1. Normal attack – He attacks with heads just like the wither, but here they have bigger explosions and they will set you on fire.
2. Reinforcement – There is a small (not really) chance to spawn a Skizie when you deal damage to the Skizzik.

Spawning of the Skizzik:

The Skizzik doesn't spawn naturally (Probably only for now.) Here is how to spawn him:
You will need: 1x Command Block, 3x Redstone Blocks, 4x Skizzik head, and 1x Skizzik head with gems.
To craft the command block, you will need to surround a redstone lamp with 8 light gray terracotta
To craft the Skizzik head you will need to smelt a wither skeleton head
To craft the Skizzik head with gems you need to get 5 rainbow gems and cut them in a stone cutter to make them different colored gems. After that cut the orange one. After that get a Skizzik Head and follow the recipe from the Recipe Book
And finally, you can see how to spawn the Skizzik in Images. (Note: The Skizzik Head with gems should be the last block you place.)

Notes:

He is immune to fire, cactus, drowning, lightning, and potions.
The Skizzik is an undead mob and won't get attacked by the wither, but he and his Skizzies will probably attack him.

Skizzies:

Types of Skizzies:
1. Normal Skizzie – This is small wither with one head. It will come near you and attack, dealing 4 hearts of damage. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, potions, and arrows. The Skizzie will drip lava. If you touch it, it will ignite you. When it dies lighting will strike on the place.
2. Ka-Boom Skizzie (Originally suggested by jacso 10) – This is small wither with one head and a creeper face. If you touch it, it will blow up. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, and potions. The KB Skizzie will drip lava too!
3. Minigun Skizzie - This is small wither with one head and a giant mouth. If will do ranged attacks with his magical minigun. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, arrows, and potions
4. Witch Skizzie - This is small wither with one head and a black witch hat. If you touch it, it will give you weakness and wither. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, arrows, and potions
5. Corrupted Skizzie - This is a small wither. If you touch it, it will give you corruption. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, and potions. It will set every (almost) blocks under it to corrupted ones.
6. Friendly Skizzie, Friendly Minigun Skizzie – Same as their normal variations, but kill monsters instead and don't fight you. You can ride them.
8. Friendly Witch Skizzie - Same as its normal variations, but kill monsters instead, don't fight you, and give you strength and instant heal.

Note:

You can tame Skizzies by right-clicking them with water bucket. You have 30% chance to get stone Skizzie instead of friendly Skizzie.
You can revert friendly Skizze back to normal by right-clicking it with lava bucket. You have 30% chance to get stone Skizzie instead of normal Skizzie
You can convert normal Skizzies to other types of normal Skizzies. Right-Click them with a specific item. (Skizzik Head = Skizzie, Gunpowder = KB Skizzie, Bow = Minigun Skizzie, Any Potion = Witch Skizzie, Corrupted Block = Corrupted Skizzie)
You can convert friendly Skizzies to other types of friendly Skizzies. Right-Click them with a specific item. (Skizzik Head = Skizzie, Bow = Minigun Skizzie, Any Potion = Witch Skizzie)

Advancements:
New:
1. Skizzik – Parent of all Skizzik Advancements.
2. Out of Ideas – Visit the Candy Plains
3. Someone overused the Rainbow Sword – Go to the ScrapLands

Old:
1. The Real Boss – Spawn the Skizzik.
2. Unbelievable – Kill the Skizzik.
3. Tame the Living Lava - Right-click on a Skizzie with a water bucket to tame it. 

Skizzik:

This is the main thing in my mod. He is based on the wither, but he has more heads and more power. He is 2 times stronger. He has more attacks and drops 10k experience on death.
The Skizzik has 500 hearts (1000 health points). He also has 5 stages and every stage he is more powerful and harder to beat.

Skizzik's Attacks:
1. Normal attack – He attacks with heads just like the wither, but here they have bigger explosions and they will set you on fire.
2. Reinforcement – There is a small (not really) chance to spawn a Skizie when you deal damage to the Skizzik.

Spawning of the Skizzik:

The Skizzik doesn't spawn naturally (Probably only for now.) Here is how to spawn him:
You will need: 1x Command Block, 3x Redstone Blocks, 4x Skizzik head, and 1x Skizzik head with gems.
To craft the command block, you will need to surround a redstone lamp with 8 light gray terracotta
To craft the Skizzik head you will need to smelt a wither skeleton head
To craft the Skizzik head with gems you need to get 5 rainbow gems and cut them in a stone cutter to make them different colored gems. After that cut the orange one. After that get a Skizzik Head and follow the recipe from the Recipe Book
And finally, you can see how to spawn the Skizzik in Images. (Note: The Skizzik Head with gems should be the last block you place.)

Notes:

He is immune to fire, cactus, drowning, lightning, and potions.
The Skizzik is an undead mob and won't get attacked by the wither, but he and his Skizzies will probably attack him.

Skizzies:

Types of Skizzies:
1. Normal Skizzie – This is small wither with one head. It will come near you and attack, dealing 4 hearts of damage. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, potions, and arrows. The Skizzie will drip lava. If you touch it, it will ignite you. When it dies lighting will strike on the place.
2. Ka-Boom Skizzie (Originally suggested by jacso 10) – This is small wither with one head and a creeper face. If you touch it, it will blow up. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, and potions. The KB Skizzie will drip lava too!
3. Minigun Skizzie - This is small wither with one head and a giant mouth. If will do ranged attacks with his magical minigun. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, arrows, and potions
4. Witch Skizzie - This is small wither with one head and a black witch hat. If you touch it, it will give you weakness and wither. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, arrows, and potions
5. Corrupted Skizzie - This is a small wither. If you touch it, it will give you corruption. It has 10 hearts. It is immune to fire, cactus, drowning, lightning, and potions. It will set every (almost) blocks under it to corrupted ones.
6. Friendly Skizzie, Friendly Minigun Skizzie – Same as their normal variations, but kill monsters instead and don't fight you. You can ride them.
8. Friendly Witch Skizzie - Same as its normal variations, but kill monsters instead, don't fight you, and give you strength and instant heal.

Note:

You can tame Skizzies by right-clicking them with water bucket. You have 30% chance to get stone Skizzie instead of friendly Skizzie.
You can revert friendly Skizze back to normal by right-clicking it with lava bucket. You have 30% chance to get stone Skizzie instead of normal Skizzie
You can convert normal Skizzies to other types of normal Skizzies. Right-Click them with a specific item. (Skizzik Head = Skizzie, Gunpowder = KB Skizzie, Bow = Minigun Skizzie, Any Potion = Witch Skizzie, Corrupted Block = Corrupted Skizzie)
You can convert friendly Skizzies to other types of friendly Skizzies. Right-Click them with a specific item. (Skizzik Head = Skizzie, Bow = Minigun Skizzie, Any Potion = Witch Skizzie)

Advancements:
New:
1. Skizzik – Parent of all Skizzik Advancements.
2. Out of Ideas – Visit the Candy Plains
3. Someone overused the Rainbow Sword – Go to the ScrapLands

Old:
1. The Real Boss – Spawn the Skizzik.
2. Unbelievable – Kill the Skizzik.
3. Tame the Living Lava - Right-click on a Skizzie with a water bucket to tame it.